Response Of Soil Organic Matter Decomposition To Soil Water Content Change In Temperate Forests

Soil organic matter?SOM?decomposition,as an important part of the ecosystem carbon?C?cycle,has a huge impact on atmospheric CO2 concentration and soil C pool.CO2 is not only the main product of SOM decomposition,but it is the major greenhouse gas.It plays an important role in the C cycle of terrestrial ecosystems and mitigating global warming.Scientists focus on the effect of warming on SOM decomposition and temperature sensitivity?Q10?through large numbers of experiments and model studies.But accompanying changes in soil moisture and its effects on soil C dynamics have been ignored.Precipitation is a direct source of soil moisture.Global warming strongly affects atmospheric circulation and water cycle,which triggers changes in precipitation pattern and soil water content.Soil water content has important influence on soil physical and chemical properties,included soil permeability and soluble organic matter concentration,microbial community structure and activity,thus affecting CO₂ released by SOM decomposition.Soil water content is determined by topographical factors,soil factors,and climatic factors in natural ecosystems.It is impossible to simulate the effect of soil water content on soil properties in the real natural environment only by indoor short-term culture.The studies on relationship between soil water content and SOM decomposition has neglected the effect of actual water content at the sampling site.In addition,few studies have analyzed the effects of different soil water contents on microbial respiration from the perspective of exogenous organic matter input.Therefore,the following two experiments were designed to study the effects of soil water content on SOM decomposition and Q10 and the utilization strategies of microbes to exogenous carbon addition under different soil water content.1.The transition from forest swamps to forests reflects the significant differences in soil water content caused by the long-term environment in the field.It is a typical sequence of soil water content change in natural forest ecosystems.Soils with high latitude and cold climate have accumulated a large amount of organic matter and are considered to be one of the most sensitive regions of global change in the world.Therefore,we selected three temperate forests as study areas.Based on topographical factors,three sampling points were selected from forest swamp to forest according to the difference in soil water content in each area.The decomposition rate of SOM?R_S?was measured rapidly by indoor continuous automatic temperature varying culture and test method,and the cumulative release of CO₂(R_CUM)and Q₁₀ were calculated.The results showed that R_S increased first and then decreased with the reduction of soil water content in cold temperate forests.R_CUM increased significantly with reduction of soil water content in early.Soil redox conditions mainly controlled R_S in this stage.When soil water content reduced further,R_S decreased with decrease of organic carbon content.Substrate quantity and quality dominated in the later stage.Differently,R_S gradually decreases with the reduction of soil water content in temperate forest.Furthermore,Q10 had regional differences in response to water content,which may be impact by soil substrate and physicochemical properties.2.Substrate addition provides energy sources for starved microbes and activates those microbes that are potentially active in a short time.The life activities of soil microbes are directly controlled by soil moisture,and it has a greater response to substrate addition in arid areas with high variability of precipitation.Therefore,we selected a relatively arid temperate forest soil,carried out incubation experiments by adjusting soil water content and adding?¹³C labeled glucose.Finally,we continuously and rapidly determined glucose respiration rate?Rg?and SOM respiration rate?RS?to study change in utilization strategies of exogenous substrates by soil microbes under different soil contents.Our results showed that the response intensity and duration of microbial respiration to glucose addition were different under different soil water contents.The response intensity of 70%WHC soil to glucose addition was the highest,while 10%WHC soil was the lowest.The duration of 90%WHC soil to glucose addition was the longest.R_g and R_S reached the maximum within 12 h after the addition except for 90%WHC soil.In general,soil moisture affects SOM decomposition by changing microbial activity,substrate availability,metabolite diffusion and so on.Soil microbes have different strategies for utilizing carbon sources from exogenous carbon and primary soil organic matter under different soil moisture.